1. Field of the Invention
The present invention relates to memory cards, and particularly to memory cards where batteries are used to make a backup of volatile memory.
2. Description of the Related Art
FIG. 41 is a block diagram illustrating an example of prior memory cards that use batteries. In FIG. 41, a memory card 300 comprises a memory section 301 that consists in an IC memory of SRAM, a power supply control section 302 that consists in a power supply IC that controls the power supply to memory section 301, and a primary battery 303 for a backup. When memory card 300 is connected to a host system apparatus 305, an external power supply Vcc is provided to power supply control section 302 by host system apparatus 305. Power supply control section 302 supplies the source voltage Vcc provided by host system apparatus 305 directly to memory section 301 regardless of primary battery 303. When a backup of data in memory card is made by primary battery 303, the voltage of primary battery 303 becomes the source voltage for memory section 301 to retain data stored in memory section 301.
As described above, prior memory card 300 has to replace primary battery 303 with a new one, before the voltage of the primary battery 303 declines. Therefore, when using memory card 300, one must care about battery life, because data disappears if the voltage of primary battery 303 declines. Further, phenomena inherent in RAM, such as changes in data due to .alpha. rays, occur during a backup of data in the SRAM, so that soft errors occur with a certain probability.